1. Field of the Invention
The present invention relates generally to the field of control of HVAC (heating, ventilating, and air-conditioning) systems. More particularly, the present invention relates to a method and apparatus for providing adaptable control of HVAC systems wherein the system changes its control behavior in response to input concerning the environment provided by a room occupant. The term HVAC system as used herein is meant to describe an environmental control system which performs any combination of heating, ventilating, and/or air conditioning functions.
2. Discussion of the Related Art
The fundamental goal of HVAC systems is to make room occupants comfortable. To achieve this goal, an HVAC system controller should regulate the HVAC system such that the room occupant feels thermally comfortable. Thermal comfort is defined by ASHRAE (see Environmental Conditions for Human Occupancy, ANSI/ASHRAE Standard 55-1981) as that condition of mind in which satisfaction is expressed with the thermal environment. Obviously, thermal comfort can be measured directly by asking the occupant how he or she feels. But thermal comfort can also be estimated objectively because of its dependence on measurable quantities of the thermal environment. Thermal comfort is primarily dependent upon whole-body thermal sensation, which at steady-state conditions is a function of the following six variables: air temperature, humidity, air velocity, clothing insulation, bodily heat production rate, and mean radiant temperature. Mean radiant temperature is defined by ASHRAE (see Environmental Conditions for Human Occupancy, ANSI/ASHRAE Standard 55-1981) as the uniform temperature of a radiantly black enclosure in which an occupant would exchange the same amount of radiant heat as in the actual environment.
Many HVAC systems use air temperature regulators (i.e., thermostats) to control the thermal environment and require that the room occupant adjust the set point (for example, air temperature) whenever he or she is uncomfortable. However, thermostatic control of an HVAC system only takes into account one of the variables that affects thermal comfort. The system must rely heavily on the supervisory role of the user. Whenever any of the other five variables change significantly, the room occupant may experience discomfort, and must determine a new set point air temperature to compensate for the change in the environment. Consequently, an air temperature regulator does not achieve the goal of controlling thermal comfort, since the room occupant may experience uncomfortable conditions under thermostatic control. Furthermore, air temperature regulation control suffers from a translation problem in that some occupants cannot effectively utilize the controls to compensate for changes in the environment. For example, when changing the set point air temperature in an HVAC system controlled by an air temperature regulator, the user ideally must assess his or her own thermal comfort and adjust the air temperature set point such that the correct environmental variable is changed in a manner that results in increased comfort. It is doubtful that the majority of occupants really try to do this. Furthermore, translating between changing air temperature reference and the effect this change will have on the remaining five variables which affect thermal comfort is difficult for most room occupants to contemplate.
Recently, controllers for HVAC systems have been developed that utilize somewhat more knowledge of thermal comfort. These controllers involve the use of a comfort index as the controlled output, rather than air temperature. A comfort index is used to predict a room occupant s thermal sensation rating of the environment based on one or more environmental variables such as temperature, humidity, or air velocity. A comfort index is used to calculate a predicted thermal sensation rating which rating corresponds to a particular thermal comfort level. One example of such comfort index regulators are HVAC systems which use Effective Temperature (ET*) as the controlled output. Effective Temperature is defined as the uniform temperature of a radiantly black enclosure at 50% relative humidity in which an occupant would experience the same comfort, physiological strain, and heat exchange as in the actual environment. A comfort index regulator based on ET* is described in Thermal Comfort Control for Residential Heat Pump by H. Itashiki, IIF-IIR Commissions, Purdue University, 1988. Another example of an HVAC control system which uses a comfort index is a system which uses Predicted Mean Vote (PMV) as the controlled output. Predicted Mean Vote is a comfort index that predicts the average thermal sensation rating of a large population when air temperature, mean radiant temperature, humidity, air velocity, clothing insulation, and rate of bodily heat production are known. See, for example, Humidity and Predicted-Mean-Vote-Based (PMV Based) Comfort Control by J. W. MacArthur, ASHRAE Transactions, vol. 92, Part 1B, pp. 5-17, 1986 and The Development of PMV-Based Control for a Residence in a Hot Arid Climate by D. G. Scheatzle, ASHRAE Transactions, vol. 97, Part 2. Predicted Mean Vote gives the thermal sensation of an "average" occupant on a bipolar psycho physical rating scale.
Although HVAC system controllers that use a comfort index as the controlled output are better able to make room occupants thermally comfortable than controllers based only on air temperature regulation, the performance of comfort index regulator controllers is limited by four important factors. A first limitation to using thermal indices for control is that they predict the thermal sensation rating of the "average" occupant. The parameters of the indices such as ET* and PMV are fit to the statistical mean of a large population. However, it is well-known that people are not alike. Values of the parameters of a comfort index which predict the thermal sensation rating of one occupant will be different than the values of the parameters which predict the thermal sensation rating of another room occupant. For example, under certain conditions, women are more than twice as sensitive as men to a change in temperature (see Thermal Comfort (Thermally Neutral) Conditions for Three Levels of Activity by P. E. McNall, Jr. et al., ASHRAE Transactions, vol. 73, 1967, pp. I.3.1 I.3.14). No mechanism is provided in prior art comfort index controllers to accommodate occupants who do not not fit the "average." This limitation of prior art comfort index regulators means that these controllers for HVAC systems are not "adaptable;" that is, they cannot adjust their control response to the requirements of different room occupants. In the present disclosure, the term "adaptable;" is meant to describe a control system which is able to modify its control capability, if needed or desired, to conform to differing occupant needs or requirements.
A second limitation of prior art comfort index regulators is that, in general, comfort indices such as PMV or ET* are implicit functions of the variables affecting thermal sensation. An implicit function is one which is defined by an expression in which the dependent variable and the one or more independent variables are not separated on opposite sides of an equation. Therefore, the values of these indices must be computed iteratively; that is, the operations required to find a solution to the function are repeated and each replication of the cycle produces results which approximate the desired result more and more closely. This is a problem for real time control systems, particularly if the iterative solution is not unique. Computing iterative solutions can be a relatively time consuming, computationally intensive procedure. The existence of only iterative solutions also makes control system design based on such indices difficult since design procedures generally rely on an explicit input output relationship.
A third limitation of prior art comfort index regulators is that the value of the parameters of clothing insulation and rate of bodily heat production must be known exactly to compute an accurate estimate of thermal comfort. As a practical consideration, measuring these parameters is generally inconvenient, if not impossible under certain conditions.
A fourth limitation of prior art comfort index regulators is that the measurement of the environmental variables, such as air temperature, mean radiant temperature, humidity, and air velocity must be made adjacent to the occupant in order to assure accuracy. This is not practical in most typical HVAC applications.
Therefore, an object of the present invention is to provide a comfort index which is an explicit, linearly parameterized function which represents the non-linear behavior of environmental variables, and is relatively independent of sensor location.
Another object of the present invention is to provide a controller and method of operating the controller for an HVAC system which uses a comfort index as the controlled output and provides adaptable control.
Still another object of the present invention is to provide a controller and method of operating the controller for an HVAC system such that the controller learns the environmental conditions that make a particular room occupant thermally comfortable.
Still another object of the present invention is to provide a controller and method of operating the controller for an HVAC system which uses a comfort index as the controlled output and is adaptable by changing the value of the parameters of the comfort index according to the thermal sensation ratings provided by the room occupant.
Yet another object of the invention is to provide a controller and method of operating the controller for an HVAC system which uses a comfort index as the controlled output, predicts a thermal sensation rating of a room occupant, and varies the value of the parameters of the comfort index in response to an actual thermal sensation rating provided by the room occupant in real time.
Yet another object of the present invention is to provide a controller and method of operating the controller for an HVAC system which responds to a thermal sensation rating provided by a room occupant to learn the environmental conditions that provide neutral thermal sensation to a particular room occupant.